From: AAAI-86 Proceedings. Copyright ©1986, AAAI (www.aaai.org). All rights reserved.
Representing
Actions with an
Assumption-Based
Truth Maintenance
Paul
H. Morris
and Robert
IntelliCorp
El Camino Real
1975
Mountain
View,
ABSTRACT
A.
the
94040
following
The
Assumption-based
modeling
Truth
Maintenance
System,
introduced
by
is a powerful
new tool for organizing
a search through
a
However,
the ATMS
is oriented
towards
alternatives.
problem
solving,
and provides
no special mechanisms
for
We describe
an approach
to
actions
or state
changes.
the ATMS to the task of representing
the effects of actions.
context
extends
traditional
tree-structured
approach
The
mechanisms
to allow context
merges.
It also takes advantage
of the
underlying
ATMS
to detect
inconsistent
contexts
and to maintain
Some results
are presented
concerning
possible
derived
results.
questionable
merges
in
treatment
of
approaches
to
the
sections,
we give a functional
overview
of the
We then describe
the underlying
representation
in terms of the ATMS.
Special
attention
is given to the situation
where
a world
has multiple
parents.
This
is followed
by a
discussion
of non-monotonic
reasoning
about
actions
in a more
KEEworlds
de Kleer,
space
of
inferential
Nado
West
California
In
System
facility.
general
TMS setting,
suggested
by the worlds
with some remarks
about related systems.
mechanism.
We close
applying
Finally,
circumstances.
the
analysis
of actions
in terms
of a truth
2. Worlds
The
basic
world
together
ordered
the
need
for
a more
elaborate
maintenance
system
suggests
treatment
of contradiction
in such systems
than exists at present.
Assumption-Based
efficient
Truth
Maintenance
System
(ATMS),
new tool for organizing
an
[2], is a powerful
By explicitly
through
a space
of alternatives.
by de Kleer
search
recording
the dependence
choices, a truth
maintenance
across
different
of
branches
the
system
of the
reasoning
steps
is able to share
search
on individual
partial
results
In effect, knowledge
gleaned
in one context
is automatically
transfered
to other contexts
where it is relevant.
The ATMS
permits
simultaneous
reasoning
about
multiple,
possibly
conflicting
contexts,
avoiding
the cost of
context switching.
The
ATMS
purely
as
presently
inferential.
constraint
temporal
This
satisfaction
changes
constituted
problems.
or actions
de Kleer [5] points out,
world, and this cannot
views
is an appropriate
problem
stance
solving
for a broad
for
modeling
actions
its ancestors
in the graph
Each
successor
represents
conflicting,
thus
action
be regarded
networks.
as representing
Each
partially
require
‘I... problem
be modeled
This
KEEworldsTM
of
the
by
application
somewhat
operators
the
action.
of an operator,
not
Since
an action
corresponds
to
an operator
itself, this assumption
less restrictive
than
that of STRIPS
to have fixed add and delete lists.
[8], which
and
underlying
contexts,
has
been
KEETM
ATMS
and to
-on(c,d)
implemented
(Knowledge
This
research
was
(DARPA)
reported
the official
here
are
position
supported
in part
under
contract
those
of the
or policy
+on(c,
to allow
maintain
in
Engineering
Environment
by
the
No. F30602
authors
of DARPA
and
Defense
Advanced
85 C 0065.
The
trademarks
should
or the U.S.
not
Research
views
be construed
government.
and
w3
W2
Engineering
are
tab)
the
IntelliCorp.
Agency
requires
Wl
Figure
Figure
Knowledge
an
is
of
system.*
KEE
action
a.s
class
*
KEEworlds,
The world
multiple,
ordered
solvers
[may] act, changing
the
in a pure ATMS
in which there
approach
facility
EnvironmentTM)
graph
associated
However,
problems
involving
some additional
mechanism.
h
advantage
of an
and QA4 [l]), taking
context
merges,
to detect
inconsistent
results.
in the
a hypothetical
extension
of the world’s
to include
a new subsequent
action.
may
as
A
a partially
of a world
is no way to prevent
the inheritance
of a fact into a daughter
context.”
In this paper we explore one approach
to using the ATMS
to support
the modeling
of actions.
The basic idea is to extend a
traditional
tree-structured
context
mechanism
(as in CONNIVER
derived
is a directed
network
resembles
a procedural
net of NOAH [9], or NONLIN
[lo],
where the actions
are fully specified.
We assume that the effects of
a fully specified
action can be represented
by additions
and deletions
of base facts,
so each world
has a set of additions
and deletions
associated
with
it that
represent
the actual
primitive
changes
determined
space.
provided
of actions.
as a whole
possibly
1. Introduction
The
with
network
then represents
action
network
graph
introduced
structure
acyclic
graph
of worlds.
Each
world
may
be regarded
representing
an individual,
fully specified
action or state change.
Projects
conclusions
as representing
of
2-l
The additions
and addition
table.
Those
block
These
2-l:
A Worlds
Graph
from
shows
an example
worlds
graph
at
Wl produce
at W2 represent
at W3 correspond
c to the table.
are
W2 and
recorded
assumed
Blocks
from
World
the
Notice
that
preconditions
are not
to have
been
tested
parent
W3 were constructed.
in the worlds.
blocks
world.
the initial
state,
while the deletion
the effect of moving
block a to the
to an alternative
action of moving
Only
in the
the
AUTOMATED
effects
of the
represented.
world
before
actions
REASONING
are
/ 13
To simplify
graph
the
discussion
is a tree,
parent
and
we will
as in figure
2-1,
assume
i.e.,
for the moment
each
world
has
a branch
of the tree corresponds
to a linear
Later,
we will
consider
the consequences
actions.
that
at
most
the
double
one
world
sequence of
of multiple
parents.
Observe
that
we
may
associate
each
world
with
the
state
that
results
from applying
the changes
encoded
by the world and all of
its ancestors
(in figure 2-l the states are depicted
inside the worlds).
Hence, a world plays a double role, representing
both a state change
and
a world
role:
assumption
may
action.
also
be thought
The
world
the state,
and
the
world
given
assumption
corresponds
of as the
environment,
actually
choice
the world’s
by the
or decision
all of its ancestors.
action
environment.
encoded
that
hand,
of the set of world
ATMS
itself
to
compute
the
accomplished
by having
a special
world.
This node may be thought
that
action
on the other
consists
and
a world
and
to the
The
world,
led to the
corresponds
assumptions
It is convenient
to
from
to use the
world
environment.
This
is
world node associated
with each
of as representing
the statement
occurs.
and a state.
The facts in the state will in general
be augmented
with deductions
using general
knowledge
of the domain.
Thus, the
facts that are true at a world fall into the following
three categories:
1. facts
inherited
2. direct
additions
3. deductions
In
from
keeping
at this
from
with
ancestor
world
facts
the
worlds
in 1 and 2
view
that
additions
actual
changes,
they are only recorded
that is, an addition
only occurs where
hold. and a deletion
where it did hold.
The
inherited
facts
follow
and
deletions
where they
the fact did
a principle
of
represent
are effective,
not previously
inertia
(essentially
STRIPS
assumption
[ll]): a fact that is added at a world
to be true in succeeding
worlds,
up until (but not including)
where
it is deleted.
The
deduced
representing
is
the
continues
a world
marked
as
reasoning
useful
facts
may
include
a contradiction.
in
The
inconsistent.
such
worlds
to do meta-level
the
A world
system
(however,
reasoning
distinguished
where
it
about
fact
FALSE
node’
FALSE,
can be deduced
generally
avoids
is possible
and
inconsistent
Figure
further
sometimes
The world
worlds).
Before
the
in ATMS
discussing
how
underlying
ATMS,
the worlds
we
graph
give
a
mechanisms
that are used, primarily
reader is urged to consult
de Kleer
the ATMS.
is implemented
brief
sketch
in terms
of
the
of
nodes
ATMS
to establish
terminology.
[3, 4, 51 for a full description
The
of
The
basic
elements
used as an
propositional
nodes,
of the ATMS
in the ATMS
By
nodes.
An
tracing
be justified
back
ultimate
justification
support
for
a
it may also have multiple
environments.
The set of
Computing
environments
for a node is called its label.
of nodes is one of the major activities
of the ATMS.
primary
transaction
This
causes
justification.
recomputed.
contradiction,
justifications.
called
nogoods
Environments
the ATMS
labels
of
There
is a special
which
is similar
The environments
and
that
of nogoods,
are not
that
the
constitute
are
discovered
used for further
is adding
nodes
to
a
be
element
called FALSE,
denoting
may
have
and
to
a node,
that
would
be in its label
are
minimal
are removed
supports
affected
inconsistent
to be inconsistent,
from
the
labels
environments.
i.e.,
of nodes
that
are
so that
world
has two ATMS
1-i / SCIENCE
world
node
a single
Node
justification
of the
environment,
adding
a fact
a justification
for the
parent,
and
Thus,
of the form
A,
is the
world
described.
F at a world can now
in terms of the world
possibility
is included.
of later
a nondeletion
deletion,
the justification
be accomplished
node.
However,
by
to
assumption
has the form
NwAA,w - F
where
A,,
is the
world
(to
subsequent
A,
allow
world
A A,,
is supplied
Wl.
-
This
the
assumption.
each
separate
A distinct
addition
If
F
where
Awl
resulting
scheme
justifications
is the
from
for addition
supplied
world
and
deleted
at
a
at
a
assumption
by the
for
of this
form
is shown
system
to represent
the
facts,
which
a fact
deletion
deletions,
and justifications
for world
installed
by the user to represent
primitive
is
justifications
additions
and
be justifications
as the presence
determination,
nondeletion
of
FALSE
ATMS,
call nogoods
deletion nogoods.
figure 3-2.
from
for
independent
deletion).
WI, the justification
to the
We will
Apart
nondeletion
is required
in
nodes, there will
deductions
from
These deductions
need be performed
only once
of the justifications
in the ATMS allows the efficient
via label propagation,
of which derived
facts hold in
worlds.
reasoning.
Derivations
Each
for World
a single
of the given world.
This
is depicted
graphically
in
It is not difficult
to see that this results
in all world
assumption
it
they
the
the
of other
of a node as a set of assumptions.
Such a set is called an
Since
a node
may
have
multiple
for the node.
supersets
determine
through
in terms
structure,
The
to
may
derivation
environment
derivations,
(minimal)
the labels
is possible
and
to a decision
or choice, and is
Nodes
correspond
to
descriptor.
Justification
is given
NW
is the
having
Directly
supplying
corresponds
elementary
context
facts or data, which
or assumptions.
are assumptions
N,
assumption
figure
3-1.
allow
assumption
3-l:
N,,
N,&AW+
where
3. Worlds
node,
entities
associated
with
it, reflecting
its
determine
of FALSE
inconsistent
arising
worlds,
from
user
representing
justifications
dead
ends
are used
in the
to
search.
non-deletion
assumption
-on(c,d)
+on(c,tab)
assumptqon
Figure
3-2:
Addition
The
nogoods
determined
nondeletion
assumptions
However,
only the
wish these to take
further
in section
a feedback
loop
5).
Thus,
that
the
by the ATMS
in addition
to
latter
represent
all the “blame”
To
in
test
with
the
feedback
only
are,
ensures
left
contain
that
nondeletion
assumptions
is necessarily
one
nondeletion
then
checked
unique
of
applied,
assumption).
as true
environment
When
as are consistent
in the world.
at
extended,
the
of the fact
W2
includes
with
changes
examples
environment
example,
the
world
also
contains
in figure
on
in an ambiguous
of
such
merges.
the
the
order
merge.
In
in
cases,
branches
in
which
In figure
both
two
arises
4-2,
the
all
that
the
they
are
we show
state
at
W5
Thus, on(c,d) is true
assumption
for the
the
the
Figure
depends
definition
now
consider
the
multiple
parents:
we
perform
merges
allows
more
call
components,
As
recombined.
Thus,
are
the
to the table.
a simple
union
which
before,
combined.
state
to
the
be
where a world
The ability
decomposed
into
has
to
nearly
and
the
In
is a
figure
in
4-1,
the
to W4 will
to stress
facts
a merge.
can be worked
on separately
the changes
represented
by
corresponding
We wish
of
situation
a world
a problem
independent
worlds
complex
such
the
that
a merge
parent
+P
-P
-P
+P
for
4. Merges
as
depend
along
a difficulty
from
the nondeletion
There
moved
changes
changes
2-1,
assumption
world
have
W4
both
is not
worlds,
blocks
the same
but
rather
4-2:
Ambiguous
on the order
of the preceding
are basically
two
is to forbid
merge.
the
of the
If so,
addition
of on(a,b)
at Wl
cannot
be consistently
added
to
environment
at W2 because
it shares
a deletion
nogood
with
world assumption
for W2. Hence on(a,b) fails to be true at W2.
ancestor
4-1,
a union
generally,
may
resulting
from
it (the
environment.
For
assumption
for the addition
of on(c,d) at Wl.
at W2.
Note,
however,
that
the nondeletion
later
More
since each nogood
contains
at most
The extended
world environment
is
to see if it is a superset
is regarded
world
We
of figure
A Merge
of
procedure
ones
effect
two
minimality
nogoods
example
independent.
a fact holds in a world,
we can compare
each
The
in the node label with the world
environment.
is done as follows
(in principle;
the actual
algorithm
is
The world environment
is extended
but more efficient).
extension
Wl.
the
nogoods
with the
are subsets of the
Deletion
In the
are
incorporates
resulting
whether
equivalent,
with as many
the fact
reduced
nogoods
accordance
removed.
the
process;
are
system
4-l:
assumptions.
environment
comparison
the
worlds
are
course,
exempt
from this
that
the deletion
nogoods
nondeletion
the multiple
so,
produces
the state
the ancestor
worlds.
choices
in the search,
and we
for dead ends (we discuss
this
in the ATMS
These
removed.
latter
Figure
Justifications
may,
however,
contain
the world
assumptions.
installs
nondeletion
assumptions
and
original
ones,
requirement,
and Deletion
the merge
of the
merge
ways
possible
to adopt
an intermediate
and further
specifying
others.
changes.
of dealing
in ambiguous
so that
Merges
cases.
the ambiguity
position,
We
now
with
this
The
other
difficulty.
is removed.
forbidding
consider
One
is to refine
the
It is also
some
examples
merges
of each
approach.
We have already
deletions
at worlds
from
of
actions
fully
required
introduced
be effective
in ancestor
specified
worlds.
actions,
to be effective
the requirement
that
with respect
to the
However,
the additions
even with respect
from
additions
and
state resulting
a strict
standpoint
and deletions
to possible
states
AUTOMATED
REASONING
could
be
resulting
/
15
from
actions
merge
in sibling
linearization
then
subgraph
in which
prove
Theorem
Thus,
next
result
or deletion
is ineffective.
1: A merge
forbid
possesses
a
any
One can
that
is not
assists
in the identification
that
of such
additions
2: A graph of worlds
an addition
is ineffective
at
two
that
neither
result
worlds
in which
is an ancestor
holds
4-2 would
It is of interest
the
forbidden
merges
effective
with
addition
occurs
With
this
approach,
and
a
the
above
restriction
deletions
that
merge
constraints
is the necessity
would
to be a high
that
led
of quickly
produce
The
a
the
world,
operation.
as
to
schemes
determining
a consistent
frequency
allow
the merge
to be computed
environments.
The other constraint
of unambiguous
cases where there
the merge.
resembles
that
be
whether
since
schemes
simple
that
is
described
union
of
ATMS
is the existence
of a large core
is only one reasonable
value for
has some
for the merge.
ancestor
in every addition
that every valid
to show
Thus,
one might
appeal,
but
arises
subgraph
but
does not
as follows.
Let
of a proposed
and deletion
linearization
define
any such
In figure
W5 in the left example,
intuitive
implementation,
of the
value
(if there
is
forbidden
otherwise).
required
that
merge
being effective.
gives the same
the merge
to be this
It is
result
common
linearization;
the merge
could
4-2, this would lead to P holding
not
be
at
in the right.
5. Actions
is
definiteness,
and for contrast,
this will be cast in terms of a Doylestyle truth
maintenance
system
[S]. The general approach
we follow
is to use a form of nonmonotonic
inference
to reason
about
the
If
excessive:
one
with
it
forbids
does not
respect
to
sufficient
condition
unambiguous
guarantees
type
an efficient
are mandatory
are, in effect,
the separate
branches
of the
because each branch
deletes a
effective
weaker
merge
to admit
valid if it results
nets [lOI
. . where actions that violate each
not be unordered
relative
to each other.
above
restriction
unambiguous.
somewhat
necessary)
a potential
possible
the merges
of the other.
additions
overriding
One
US call a linearization
such
be disallowed.
that
the
are
two
here.
appear
of the other.
Indeed,
additions
and deletions
that
preconditions.
From this perspective,
networks
of figure 4-2 are in conflict
In a sense,
merges
that
are
described
A further
admits
a linearization
if and only if there are
the
for deletions.
for conflict-free
procedural
others’ preconditions
may
precondition
by
are always
Theorem
A similar
There
worlds).
in which
least
forbidden
is unambiguous.
to ancestor
actions,
might
expected
we are assuming
in figure
one
merge
an addition
criterion
(remember
respect
worlds.
of the proposed
the following.
above
The
or cousin
if the ancestor
many
require
that
non-ancestor
(but
still
merges
effects
of actions.
contradiction
not
as
can be obtained,
and NonMonotonicity
It is instructive
to consider
how actions
might be represented
more general
TMS setting,
as suggested
by the worlds system.
truth
However,
the
is somewhat
behavior
different
maintenance
systems.
will
a
we require
from
the
in a
For
in response
to
approach
in
system,
as
standard
follows.
Theorem
3: A sufficient
unambiguous
is
that
contain
two worlds,
other of which adds
the other.
This
criterion
We now
and
adopt
for
ancestor
a merge
subgraph
We
to be
may
the examples
of figure
Besides
which
then
fact,
and
the absence
ignorance,
the
not
original
merge,
fact
when
Notice
the
holds,
that
merge
irrespective
of the
order
in
Otherwise,
we are ignorant
of the
from the merge simply denotes such
of the fact
falsity.
the fact
the
are performed.
this
definition
is unambiguous.
is consistent
With
the
dual
to the pessimistic
true
in the
merge
consistent
with
optimistic
merge,
at W5 in both examples
of figure 4-2. A
merge is the optimistic
merge where a fact is
if it is true
the
in some
original
for
P is present
the
linearization.
Again,
unambiguous
at W5 in both
case.
this
With
the
examples.
deletions
is unchanged.
The
different
merge,
the
descendant
effective
with
merge
tends
defensible
One
16
/
deletions
additions.
respect
to
be
on semantic
might
imagine
SCIENCE
are
effective
For
the
to
easier
with
implement
are obtained
deletions
case,
additions
affect,
the
to
the
only
i.e..
pessimistic
all except
deletions
(the
efficiently,
are
optimistic
although
less
grounds).
a wide
variety
of possible
For
merge
algorithms.
state
about
facts
context
b”.
of
an
of
the
to a particular
in the
records
Note
that
of past
in time.
“present”
past
actions
there
may
such
like
as
A3:
be several
same
description;
we
giving
them
unique
the identifiers
is not
so far - the relative
can
action
occurrence
point
be
actions
represented
by
to present
facts.
u was not
we justify
moved
P5 as
example,
A3 A I% + block
a is on block
(05)
Here,
-
“(D5)”
it a nonmonotonic
justification
of the form
F5
indicates
that
a subsequent
b.
condition
of the form “block
In order to allow deletion,
A3.”
off block b after
an assumption
by giving
statement
of merge
For
respect
optimistic
ancestor
to
kinds
additions
the
are entered.
current
of individual
between
the
effects
The
positive
justifications
linking
the
is
We now discuss the effect of merges on the ATMS representation.
When a world has multiple
parents,
the justification
for the world
node must be expanded
to include
each of the parent
world nodes
among
the justifiers.
The justification
scheme
for additions
and
by different
selections
of which
which
justifications
for FALSE
assertions
P5 is a preservation
P is absent
or
of a situation
actions
with
the
occurrences
by
The numbering
of
identifiers
such as A3.
Thus intended
to imply
temporal
order.
timing
of past actions has not been represented.
with
pessimistic
context,
evolution
containing
occurrences
distinguish
4-2.
other
approach
to removing
criteria
for defining
the
be assured
the actions
the
a is on block b,” the
a was placed on block
“block
“block
the ambiguity,
merge.
In the
pessimistic
merge, an individual
fact belongs
to the merge
if it
survives
in every linearization
of the actions.
The rationale
is that
we may
regard
describing
not
one of which deletes a fact and the
it, such that neither
is an ancestor
of
also prohibits
follow
the
additional
condition
the
that
“some
D5 is an OUT-justifier,
action
action,
say
where
D5 is the
after A3 moves block u off block 6”.
the block
off, we supply
A4, moves
If
a
justification
A4-+05
causing
the OUT-justifier
to come IN, thereby
undercutting
t,he
derivation
of “block
a is on block b.” Note that the information
about
the
these
A
difficulty
solving
the
relative
timing
of actions
is now implicitly
represented
by
justifications.
with
process
search
space.
this
generates
We
representation
contradictions
do not
wish
arises
that
the
when
represent
preservation
the
dead
problem
ends
assumptions
in
to
be implicated
in these; rather,
we wish the assumptions
representing
choices of actions
to be the ones considered
for revision.
Choosing
a
as culprit
during
backtracking
would
preservation
assumption
amount
to
postulating
the
deletes
one of the facts
make
the
maintenance
the
TMS,
has
ensure
that
have
separation
between
by de Kleer,
only
informed
the
like
directly,
Incidentally,
identification
but
a
then
exist
Some
handles
an
unknown
contradiction.
problem
that
it about.
TMS
something
refuted
actions
of
to the
suggested
the
solver
existence
leading
this
solving
from
are
the
“sheltered”
that
One
not indirectly
in response
of
problem
is required
to
to a contradiction.
the need for a more discriminating
is not confined
to the difficulty
system
suggests
a
schemes
The
approach
KEEworlds
possibility
is to
which
could
be
assumption,
TMS
maintenance
shortcoming
handle
efficient
tool
KEEworlds
existing
described
facility
for
is to
consequence
defeat
his
assumption
standard
break
in
of waking
a window.
However,
the occupants,
purpose.
Let
if they
us suppose
the
this
may
are home,
burglar
have
which
makes
the
“contradiction”
of waking
the occupants,
might
elect to revise the
assumption
that the occupants
are home, even though
that is not
subject
to the burglar’s
control,
instead
of the real culprit,
breaking
the window.
The system
would
in effect
regard
the undesired
of waking
the
occupants
However,
it is only when
there
occupants
being
absent
that
this
This example
of “wishful
thinking”
systems
in general
need
as evidence
for
their
refined
treatment
outlined
here could
PI
frame-based
existing
implemented
the
exploration
during
either
to
as
provide
multiple
multiple
representation
truth
of
system
provides
the
and
The
situations.
worlds
system,
part
a useful
with
an
a graphical
of worlds
and allows
rule-based
forward
or backward
chaining.
discussion
of the
K.EEworlds
of its use, may be found in [8].
Bobrow,
facility,
G. and B. Raphael.
New Programming
Languages
for Artificial
Intelligence
Research.
Surveys
Computer
PI
PI
absence.
is independent
evidence
for the
possibility
is worth
considering.
suggests
that truth maintenance
a more
way
References
default
that the occupants
are home.
The difficulty
is that a
truth
maintenance
system,
in attempting
to resolve
the
consequence
the
appears
about
integrates
An
application-oriented
together
with an example
would
the
been
and
reasoning
facility
browser
for manual
generation
of worlds
process of culprit
with
preservation
has
of I(EE
assumptions.
As another
example,
consider
a situation
where
a
burglar
is planning
to break
into
a house
late
at night.
To
accomplish
his purpose,
he must choose some method
of entry.
One
method
in
contradictions.
truth
of view
the
mechanism
correctly.
that
if we
and
point
those
new
action
However,
141
de Kleer, J.
Choices Without
In Proceedings,
1974.
Backtracking.
Austin,
AAAI-84.
Texas,
de Kleer, J.
An Assumption-Based
Truth
Maintenance
Artificial
28(l),
1986.
28(l),
198G.
de Kleer,
Intelligence
1984.
System.
J.
Extending
Artificial
of contradiction
6(3):153-174,
the ATMS.
IntelEigence
handling.
i51
Although
the approach
ATMS more directly
for modeling
actions,
for a user to have to input
the justifications
and
deletions
by hand.
provides
a framework
that
an action
modeling
be adapted
to using
The worlds
facility
described
earlier
presents
a more convenient
interface
to
PI
which
graphs
items
with
are recorded
in data
the
same
apparent
different
results
for a merge.
apparently
have
no notion
mentioned
McDermott’s
justifiers.
by McDermott
approach
However,
Boolean
equations,
pools.
external
This
means
that
structure
may
Another
difference
of contradiction
in the
is that
this
requires
rather
than
paper).
One
justifications
that
the
labels
simple
PI
two
have
is that data pools
(at least
none
is
be computed
PI
of
illI
ViewpointsTM
appears
quite
described
comparisons
here.*
since
the underlying
facility
similar
of Inference
in behavior
However,
it
little information
mechanisms
is
changes.
terms
of
System.
A realization
an
An
underlying
examination
*
Viewpoints
and
difficult
has been
to
made
ARTTM
facility
make
detailed
available
about
of ART.
LVe have
described
an approach
mechanism
that represents
a partially
state
Corporation’s
to the worlds
to constructing
ordered
network
of the mechanism
Assumption
of a similar
ART are trademarks
of Inference
Based
has been
Truth
representation
a context
of actions or
described
1986.
System.
Intelligence
12(3),
1979.
Dependencies:
Nardi, B. and A. Paulson.
Multiple
Worlds
With Truth
ECAI-86.
A Synthesis.
Publishing
Sacerdoti,
and
Maintenance
Brighton,
Nilsson,
N. J.
Principles
of Artificial
England,
Machine
In AI Applications.
1986.
Intelligence.
Company,
Palo
Alto,
Ca.,
1980.
E.D.
A Structure
Elsevier
the ATMS.
The
system
Maintenance
McDermott,
D.
Contexts
and Data
Tioga
ilO1
by solving
procedure
28(l),
J.
In Proc.
attractive
aspect of
may
have
OUT-
propagation
the ATMS.
IEEE Transactions
on Pattern Analysis
Intelligence
5(3):237-246,
May, 1983.
of
data pool
order
in
with
Intelligence
Artificial
Remarks
pools
Doyle,
A Truth
system.
The
worlds
considered
here
resemble
the
data
h?cDermott
[7].
H owever,
the result
of a merge in the
approach
is determined
by the arbitrary
chronological
J.
Solving
Artificial
it would be cumbersome
representing
additions
PI
6. Closing
de Kleer,
Problem
the
for
Plans
North-Holland,
Tate, A.
Generating
Project
Tn I.1C,-U-77,
and Behavior.
1977.
Networks.
pages
888-893.
Cambridge,
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[12]
lj’aldinger,
Achieving
In Elcock,
pages
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E. and Michie, D. (editor),
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in
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in a classical
Corporation.
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REASONING
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